The present invention relates generally to multi-level voltage sources used to power electric devices, and, particularly, to a multi-level Pulse Width Modulated voltage source used to power electric motors used in seagoing vessels.
The Navy has committed to the development of the xe2x80x9call electric shipxe2x80x9d and has compared the impact on ship operations with the changing from sail to steam and from fossil to nuclear power. The heart of the all electric ship is the so-called xe2x80x9cintegrated power systemxe2x80x9d (IPS), which includes the propulsion motor subsystem.
In a seagoing vessel, such as a surface ship or a submarine, the propulsion motor is important to achieving performance objectives, especially in the area of acoustics. In order to avoid so-called xe2x80x9cpassive detectionxe2x80x9d by enemy ships, submarines, aircraft, and land-based detection and tracking systems, the energy associated with the acoustic signature of Navy vessels should be minimized and dispersed.
Electric motors in Navy vessels may be driven with Pulse Width Modulated (PWM) inverters to control motor speed, output torque, output power, and other parameters. Several challenges are presented by the use of PWM voltage sources to power electric motors.
For example, a challenge presented by the use of PWM voltage sources to power electric motors relates to the issue of waveform distortion. In particular, harmonic distortion in the PWM voltage waveform (referred to herein as the xe2x80x9charmonic distortion problemxe2x80x9d) can significantly degrade electric motor performance. For example, harmonic distortion can increase high frequency noise, thereby degrading acoustic performance of the electric motor.
Harmonic distortion can also increase heating of the motor, thereby causing breakdowns and reducing reliability (e.g., mean time between failures [MTBF]) of the electric motor. Significant heating can also adversely affect other equipment or components near the electric motor.
Harmonic distortion can also increase vibrations in the motor. These vibrations represent a power loss, thus reducing the electromechanical efficiency of the system.
Typically, PWM voltage sources are PWM inverters that provide an output voltage waveform having a series of voltage pulses that can have several discrete levels. For example, FIG. 1 shows a simulated 5-level PWM voltage waveform with a step size of 0.5 volts. The five output levels are +1, +0.5, 0, xe2x88x920.5, and xe2x88x921 per unit. The smooth sinusoidal curve is reference current 100. The slightly noisy sinusoidal curve overlaying the smooth sinusoidal curve is the PWM current 150. The stepped curve is the PWM voltage output 175. As can be seen, the PWM voltage output 175 generally tracks the magnitude and shape of reference current 100.
The relatively granular step size and the low number of output levels means that the sharp departures between pulses will cause harmonics to be present in the voltage input to the electric motor. These harmonics can cause the efficiency, reliability, and noise performance problems discussed above.
Some commercially available inverters used for industrial motors use a 3-level H-bridge cell. These inverters provide 3-level PWM voltage outputs, usually at less than 600 volts. With the small number of levels and the large step size, these 3-level H-bridge cells will lead to many of the difficulties discussed above when waveform fidelity is important to satisfactory system performance.
It has been proposed that a multi-level (greater than three) inverter could be constructed using two or more (n) cascaded or stacked 3-level H-bridge cells. For example, this inverter could use n voltage inputs at the same voltage level in order to provides n times the voltage level of a single 3-level H-bridge cell. Such an inverter could provide at least 5-level PWM voltage output levels at much greater than 600 volts.
The interested reader is referred to Hammond, U.S. Pat. No. 5,625,545, entitled xe2x80x9cMedium Voltage PWM Drive and Method,xe2x80x9d Opal, et al., U.S. Pat. No. 5,638,263, entitled xe2x80x9cLow and Medium Voltage PWM AC/DC Power Conversion Method and Apparatus,xe2x80x9d and Duba, et al., U.S. Pat. No. 5,933,339, entitled xe2x80x9cModular Static Power Converter Connected in a Multi-Level Multi-Phase, Multi-Circuit Configuration,xe2x80x9d for discussions relating to the above approach. Each of the aforementioned three patents is herby incorporated by reference in its entirety.
It has also been proposed that an n-cascaded 3-level PWM H-bridge cell voltage inverter could receive different DC voltages, rather than the same voltage for all n cells. Such an approach may permit an increase in the number of the PWM voltage output levels, such as up to 9 levels for an n=2 device Just by way of example, the DC input voltage on one cell could be double or triple the DC input voltage as the other cell in an n=2 configuration.
The interested reader is referred to Mueler, et al., U.S. Pat. No. 5,734,565, entitled xe2x80x9cReducing Switching Losses in Series Connected Bridge Inverters and Amplifiers,xe2x80x9d and Lipo, et al., U.S. Pat. No. 6,005,788, entitled xe2x80x9cHybrid Topology for Multi-Level Power Conversion,xe2x80x9d for discussions related to the aforementioned approach. Each of the aforementioned two patents is hereby incorporated by reference in its entirety.
One reference in the literature touches upon the concept of connecting two five-level cells in series. The interested reader is referred to M. R. P. Kumar and J. M. S. Kim, xe2x80x9cDeadbeat Control of Hybrid Multilevel Switching Converter,xe2x80x9d Proceedings of the IEEE Power Electronics Specialst Conference, Vol.1, pp. 782-788, January 1996.
In order to provide a contextual backdrop, the aforementioned has focused on problems and applications in the military context. However, it should be unders tood that these issues exist in non-military commercial and other applications where PWM voltage sources are used to drive electric motor s and other electrical devices.
An embodiment of the present invention comprises a multi-level PWM inverter that is constructed by cascading multi-level H-bridge inverters and providing different voltage inputs to the multi-level H-bridges. The values of the voltages, or the ratios of the voltages, are selected in order to provide an increased number of output levels. In the cascaded arrangement, at least one of the multi-level H-bridge inverter s has more than three levels. Preferably, at least one of the multi-level H-bridge inverters is a 5-level inverter.
According to one aspect of the invention, the cascaded arrangement may use a primary 5-level H-bridge with at least one 3-level H-bridge (referred to as a xe2x80x9c5/3Hxe2x80x9d arrangement). This arrangement can provide up to fifteen output levels if regenerative voltage sources are used, and up to eleven output levels if a non-regenerative voltage sources are used. According to another aspect of the invention, the cascaded arrangement may be used to provide a rectifier output instead of an inverter output. Other types of outputs could be provided by the cascaded arrangement.
The invention can yield improved power quality and control by providing more PWM voltage levels with fewer H-bridges. Good total harmonic distortion (THD) performance is observed. The improved power quality and control can mitigate the power performance, reliability performance, and the noise performance problems associated with the matching problem, harmonic distortion problem, and the switching problem discussed above. Additionally, fewer H-bridges can result in lower cost and can enhance reliability.
Accordingly, it is one object of the present invention to overcome one or more of the aforementioned and other limitations of existing systems and methods for driving and/or controlling electric motors.
It is another object of the invention to provide a multi-level voltage source that mitigates the matching problem associated with driving and/or controlling electric motors.
It is another object of the invention to provide a multi-level voltage source that mitigates the harmonic problem associated with driving and/or controlling electric motors.
It is another object of the invention to provide a multi-level voltage source that mitigates the switching loss problem associated with driving and/or controlling electric motors.
It is another object of the invention to provide a multi-level voltage source that provides a greater number of voltage levels at a reduced part count.
It is another object of the invention to provide a multi-level voltage source that provides a greater number of voltage levels at a reduced cost, while maintaining good reliability and overall system performance.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention. It will become apparent from the drawings and detailed description that other objects, advantages and benefits of the invention also exist.